The present invention relates to a valve driving apparatus provided for a valve such as a ball valve, butterfly valve, plug valve, gate valve, etc. and adapted to cause a valve stem thereof to be rotated to thereby open and shut the valve body.
There have heretofore been proposed two kinds of valve driving apparatuses, one of which is a double-acting cylinder type valve driving apparatus utilizing the reciprocating movement within a cylinder and the other of which is a spring-return type valve driving apparatus utilizing the reciprocating movement motivated by a compression coil spring in conjunction with movement within a cylinder. Generally, the reciprocating movement is converted into a rotating movement and simultaneously transmitted to a driving shaft by a conversion mechanism of a combination rack and pinion type, a scotch yoke type or a link type and thereby the valve body provided on the valve stem connected with the driving shaft is either opened or shut.
The conventionally known valve driving apparatus of a double-acting cylinder type has the disadvantages that a continuous reciprocating movement of the piston of the cylinder and a continuous rotating movement of the driving shaft give wear to the bearing portions for the piston rod and for the driving shaft resulting in the replacement of the bearing portions with new ones in a comparatively short period of time, and that the use of the apparatus for a long period of time reduces the driving efficiency due to the formation of rust on the inner wall of the cylinder, high frictional resistance between the cylinder and the piston and, with the utilization of the scotch yoke type conversion mechanism, high frictional resistance between the pins and scotch yokes.
In the conventionally known valve driving apparatus of a spring-return type, continuous compression and extension of the compression coil spring disposed inside the apparatus gives rise to relative displacements in the rotating direction between the both ends of the compression coil spring and the both end-receiving portions and further gives rise to a twisting phenomenon of the spring which is an unavoidable characteristic feature of the spring. Assuming that one end of the spring and the portion of the apparatus receiving the one end of the spring are such that they cannot easily be slid relative to each other, the other end of the spring is given a rotating movement which is transmitted to the spring holder fixed to the piston rod. That is to say, the rotating movement of the spring holder is transmitted to the piston rod and further to the piston. This will create the possibility that the conversion mechanism disposed between the piston rod and the driving shaft be given an excessive movement or force and thereby causing disengagement between the pins and the scotch yokes in the scotch yoke type conversion mechanism, disengagement between the toothed wheels in the combination rack and pinion type conversion mechanism, etc. However, since the conversion mechanism has a construction such that no such excessive movement is made though an excessive force exerted on the conversion mechanism, attention is directed toward the aforementioned unavoidable phenomenon. That is to say, the conventionally known valve driving apparatus of a spring-return type has the disadvantages that there occurs displacement and friction between the both ends of the spring and the both end-receiving portions within the apparatus to thereby give rise to an energy loss, damage and wear, that the rotating action of the valve stem may possibly be obstructed and therefore the apparatus cannot be used with precision and stability for a long period of time for the aforementioned reasons.